add Build-time Evaluation for CSS Calculations

[jantimon]

This PR adds support for evaluating simple calculations in CSS template literals at build time, improving runtime performance while maintaining code readability.

Changes

• Added a new expression evaluator that handles:
  • Basic arithmetic operations (+, -, *, /)
  • Constant values
  • Member expressions (e.g. theme.spacing.lg * 2)
  • Parentheses for precedence control
• Integrated evaluation into the CSS template literal transformer

Examples

// Input
const theme = {
  spacing: {
    base: 8,
    lg: 16
  }
};

const Container = styled.div`
  /* Simple calculations */
  width: ${300 / 2}px;
  height: ${40 * 3}px;

  /* With constants */
  padding: ${theme.spacing.base * 2}px ${theme.spacing.lg * 1.5}px;

  /* With parentheses */
  margin: ${(theme.spacing.base + 4) * 2}px;
`;

// Output (after build)
const Container = styled.div`
  width: 150px;
  height: 120px;
  padding: 16px 24px;
  margin: 24px;
`;

Performance Impact

• Moves calculation overhead from runtime to build time
• No impact on development experience (calculations remain readable in source)
• Reduced bundle size (final CSS contains only computed values)

Notes

• Only evaluates expressions that can be fully resolved at build time
• Maintains source readability while optimizing output

Fixes #174